A known method of preparing certain foodstuffs, e.g. chicken, seafood, various kinds of meats, and the like, is to immerse these foodstuffs in heated oil, shortening or animal fat and cook the foodstuffs until done. This type of food preparation is commonly called deep frying. Establishments handling large quantities of food often resort to deep frying under pressure to meet the demand for fried food. Pressure type deep frying permits the food to be prepared in a relatively short time as compared to open type deep frying.
One problem is common to both the open type and pressure type of deep frying systems, namely, the oil, shortening or animal fat (hereinafter, collectively "oil") has a limited useful life and must be completely replaced at regular intervals. The reason for this limited life is that after a certain period of use the oil breaks down, characterized by foaming, a dark amber color, off flavor, smoking, and formation of gums and residues. The breaking down of the oil is due to the chemical formation therein of a high degree of free fatty acids. These adverse and destructive free fatty acids have been attributed to the breakdown of the chemical bond between the glycerine and fatty acids which make up the oil.
This breakdown is due mainly to hydrolysis, the presence of contaminating impurities in the oil, and sustained use of impurity contaminated oil at high temperatures. The impurities often found in the oil are derived in large part from the food being prepared. For example, chicken during deep frying will release, among other things, a product commonly called "glue". Residual blood remaining in the chicken may also be released.
In addition to the "glue" and blood, other materials such as flour, bread particles, salt, spices and the like forming parts of the covering or breading applied to the chicken will also enter into the oil as part of the total impurities. Straining and filtering the oil helps to add to the effective life. However, most filtering systems, aside from being elaborate, expensive, and awkward, cannot effectively prevent breakdown of the oil since the destructive free fatty acids are formed while the oil is being used at high temperatures to fry food. These free fatty acids remain in solution with the oil, even when cooled, resisting the straining and filtering.
Oftentimes the oil is replaced on a regular basis, usually on the basis of change in color, and is not permitted to be used until it completely breaks down. The reason for this is that the contaminated oil, if not replaced, will give the food an undesirable flavor and unattractive appearance. This is directly attributable to the high degree of free fatty acids in the oil, and the adhesion of carbon particles already formed, as well as the other suspended impurities in the foods.
In addition to causing breakdown of the oil, the impurities transfer an odor or off flavor to the oil. Such odor or off flavor (i.e. taste) being characteristic of the particular food being prepared. For example, the oil used for preparing chicken would smell like the chicken. Odorous oil cannot be used in preparing other foods due to the danger of the odor of one food being transferred to the other food (viz. fishy smelling chicken).
The rate of formation of free fatty acids due to the presence of impurities and moisture in the oil is accelerated when the oil is used at sustained high temperatures. Accordingly, the temperature of the oil is kept relatively low during cooking (e.g., 325.degree. F. to 350.degree. F.). Thus, the cooking time for the food product is necessarily lengthened as compared with oil used at higher temperatures.
As mentioned earlier, hydrolysis is a cause for oil, shortening and animal fat breaking down and is due to excessive amounts of water in the oil. An excessive amount of water in the oil or cooking medium also results in the rapid generation of steam as the medium is heated to high temperatures. This rapid generation of steam also causes violent sputtering which is dangerous to anyone who might be near an open fryer. The sputtering caused by the generated steam also carries some of the oil or other cooking medium into the surrounding environment creating waste. In order to avoid sputtering by generated steam when deep frying refrigerated foods, a common practice has been to pre-heat or blanch the refrigerated foods to eliminate some of the moisture. Aside from necessitating an additional procedure in preparing these refrigerated foods, this pre-heating is unsanitary and can result in food contamination. Some states have recognized this unsanitary danger of pre-heating refrigerated foods by prohibiting restaurants and the like from pre-heating refrigerated foods prior to cooking.
Foods with high moisture content, however, also play an important and favorable role in deep frying operations, and particularly in pressure deep frying. The steam generated from the moisture in the foods being fried circulates through the cooking medium and contacts the food to restore the lost moisture thereby resulting in a tender, moister and tastier food product. However, an additional problem arises, especially in pressure frying, in that the steam, if not properly controlled, can cause the food to become over-cooked. In the case of chicken, for example, the meat can fall away from the bones if overcooked. If the steam is generated too rapidly in open deep frying, thereby not permitted to circulate about the food, the food will tend to char since it will be getting too much heat too fast. Therefore, the temperature of the oil or other cooking medium during the cooking process cannot be raised too high.
One other problem common to both open type and pressure type deep frying is that the food product absorbs and retains a certain amount of the cooking medium which detracts from the taste of the food as well as being another source of waste. The amount of oil, shortening or animal fat absorbed by the food product increases as the length of cooking time increases. The cooking times currently used are unnecessarily long because of the necessity of using low temperatures, as indicated previously. Conscientious restaurants oftentimes use a degreaser or defatter for removing the residual fats in the food products. Degreasing adds a time consuming step to the food processing procedure, with the degreasers or defatters having the additional disadvantages of being expensive and bulky.
Pressure type deep fryers are always explosion problems, especially if operated at very high pressures such as above 35 p.s.i. Furthermore, the food product resulting from the use of the pressure fryers currently existing are not satisfactory, due mainly to the problem with water content and impurities mentioned earlier. The dissatisfaction with high pressure fryers is demonstrated by the scarcity of use of this type of pressure fryer today.
The pressure deep fry systems commonly used are also limited in the amount of food product which can be prepared in individual vessels. One process extensively used is limited in the amount of food prepared in each container to approximately five pounds. This limited capacity has necessitated the use of a large number of containers individually heated on open gas burners in order to meet the demand for food product. After the food is prepared by this process, fat laden steam is released, and the container hand carried to a strainer vat and dumped of all its contents. The fat soaked finished food product is then put into a defatter before being served. Needless to say, the just described process is inefficient, wasteful of valuable oil, shortening or animal fat, dangerous to operators who have to open the containers and carry them to the strainer vats, and very expensive to operate, among various other deficiencies.
Pressure cooking is also used for preparing other food products, such as stewed chicken, pot roasted beef, vegetables, and dough products, for example, with water being used as the cooking medium. The water is either discarded or used as gravy stock, in the case of meats, after the cooking of the products is completed. In the case of pressure cookers used to date, any contaminating foreign material on the products tends to recirculate about the cooking vessel and remain on the product, in the water, or on the walls of the vessel. In other words, there is no way for foreign material to escape from the cooking vessel during cooking.
My earlier cooking system and method of preparing foodstuffs in a heated liquid cooking medium in a pressurized vessel is described in U.S. Pat. Nos. 3,613,550 and 3,809,777. The advances made were improvements in permitting steam generated from heating the foodstuffs to be relieved at a predetermined high limit until a desired pressure is achieved and repeating that cycle until the foodstuffs are completely cooked. Upon completion of the cooking cycle and before opening the vessel, the steam pressure is used to completely discharge the liquid cooking medium from the vessel to a filtering system and then to a reservoir for reuse. Any residual steam is then vented and the vessel can be opened to remove the cooked foodstuffs. This system functioned well for its time but was eventually found to be inefficient for very small amounts of food to be cooked and for foods with a lower water or moisture content.
Further, with the ordinary pressure cooker for deep frying, it is difficult to cook small quantities of foodstuffs as well as foods with low moisture contents, e.g., hamburgers, pork, spare ribs, frozen fish patties, bacon and sausages. These low moisture foods do not give off sufficient moisture to make the pressure seal on the cooking vessel before the cooking medium cooks the outer surface of the food while the inner portions remain raw. Thus, the low moisture foods do not give off sufficient moisture to create enough steam to build the pressure necessary to seal the vessel. In turn, this low moisture content will fail to provide sufficient moist heat to fully cook the inner portions of the food product before the outer portions are overcooked.
Moreover, there were also continuing problems in the area of contaminating the cooking medium after continued reuse and high temperatures breaking down the cooking medium. However, before discussing any of my improvements for cooking foodstuffs in closed vessels using a liquid cooking medium and super-saturated steam, certain other problems must be discussed. It has been argued that elevating frying temperatures for foodstuffs alters the appearance of the liquid cooking medium, i.e. the oil, and the character of the cooked product. The main objective is to produce a low fat fried food of good appearance without adverse effect to the liquid cooking medium through continued reuse. This can only be achieved by alleviating the causes for the breaking down of the liquid cooking medium.
The principal causes of hydrolysis, the cause for the breaking down of the liquid cooking medium, are the reactions of the liquid cooking medium with water and with air. For example, the introduction of a cooking medium containing a triglyceride with water and the addition of heat will cause a chemical reaction producing glycerol and a number of fatty acids. The fatty acids remain soluble in the liquid cooking medium which has lost some of its volume through the reaction. Also, the injection of air, i.e. atmospheric oxygen, to the liquid cooking medium, in order to build pressure in the cooking vessel, will cause a chemical reaction. The fatty content of the liquid cooking medium will react with the oxygen and trace metals from the cooking vessel when heated producing hydroperoxides and various carbonyl compounds (acidic). These resulting chemicals remain soluble in the liquid cooking medium which has, again, lost some of its volume through the reaction. In both cases, the chemical impurities are retained in the cooking medium solution although strained and filtered.
I have improved the present method for elevating pressure in high and low temperature pressure cookers and, thus, eliminated the problem of breaking down the liquid cooking medium in such a way as to create unwanted and undesirable impurities. By introducing super-saturated steam into the cooking vessel from the beginning of the cook cycle, and then throughout the cook cycle at predetermined times (if desired), I have eliminated the injection of air or water into the vessel with its detrimental effects on the liquid cooking medium and on the cooked foodstuffs.